Compared to conventional, compact meat products, a foamed meat product offers a number of advantages, especially in the field of pet food. The light, smooth structure already gives the food a pleasant appearance. The increased volume and the reduced density of a foamed meat product also result in a reduction in the calorie density and the specific calorie content within a particular volume, as a result of which it is easier to maintain or reduce a given body weight. In addition, it results in a changed taste experience while eating, improved digestion and also, in the case of certain species, such as cats or dogs, which tend to a suboptimal liquid intake, it creates the possibility of improving the intake of liquid, together with the solid food.
Especially in the production of pet food products, the problem exists that it is on the one hand desirable also to use less expensive parts, such as animal by-products, as the raw material, while, on the other hand, some parts, such as in particular sinews, ligaments, twisted or coiled sheaths round connective tissue (epimysium, perimysium and/or endomysium) or similar strand-like, fibrous or filamentous collagen-containing parts of connective tissue, are very difficult to comminute and resist the comminution procedures conventionally used, which then leads to clogging and other difficulties in apparatuses of the kind conventionally used for foaming meat mixtures.
Meat and animal by-products contain not only the desirable muscle proteins (myofibrillary protein) and meat proteins (sarcoplasmatic protein), but also connective, structural and supporting tissue (stroma proteins). The most important connective tissue is collagen, together with elastin and reticulin, proteins from cell membranes and other proteins. The connective and supporting tissues comprise sinews, ligaments, fascia, skin and hide and are found in arteries, veins and separating tissues such as basal membranes (basal lamina) and also take the form of fibres and membranes which form a sheath round each individual muscle fibre (endomysium), bundles of muscle fibres (perimysium) and entire muscles (epimysium), fat cells or organs in animal tissue.
Because of the great strength and elasticity of this connective tissue—compared to the muscle tissue—, the connective tissue membranes round, in particular, sinews and ligaments (the latter mainly contain collagen of types I, III, IV, XII, XIX) are not chopped small enough during the comminution of meat or animal by-products, e.g. by cutters or mincers. Collagens, which can account for up to ⅓ of the entire body protein, are structural proteins of the skin, connective tissue and bones. As structural proteins, they determine the structure of the cell and thus ultimately the nature of the tissue and of the entire physique. After the meat and animal by-products have been comminuted in order to produce a fine meat mixture or meat emulsions, washing processes to extract the muscle tissue can reveal that these meat products contain long twisted connective tissue membranes, sinews and ligaments—which are as yet not heat-treated.
Even after the products have been comminuted by passing them through a 2 mm perforated disk or when they are emulsified through a 0.3 mm wide blade gap, it is still possible to find sinews, membranes and ligaments in the meat mixture with a total length of more than 40 mm.
During the production of meat emulsions, the muscle proteins are partially released from their matrix by the addition of salts, phosphates and other minerals. The connective tissue fibres and sinews etc. remain present in the meat mixture as coiled membranes and fibres. When this meat mixture is conveyed by positive-displacement pumps past measuring probes, mixer fingers, conventional static mixers or flow dividers installed transversely to the direction of flow, it is found that these fittings gradually become clogged by the longer fibrous sinews, twisted membranes and/or connective tissue fibres. If these fibrous parts from the meat mixture continue to collect on the fittings, it is even possible for entire pipes to become blocked.
The presence of sinews, ligaments or twisted and connective tissue membranes of this kind in meat emulsions produced on an industrial scale is probably also the reason why right down to the present day, no industrially homogeneously foamed meat products have become established on the market. This is the reason why in the patent literature (e.g. JP 2219562 A, U.S. Pat. No. 6,475,551 B2) on the foaming of meat products, either only silent cutters, bowl choppers or pressurized gassing are used, because these processes are not excessively disturbed by sinews, ligaments and membranes. The foam structure obtained with these processes is inhomogeneous, however, and is not stable. It is not possible to produce fine, stable foams from highly viscous liquid foams in this way.
The processes mentioned therefore only use meat with the sinews removed, or pure muscle tissue (lean meat). Formulations consisting of high-price materials of this kind and containing no sinews or long connective tissue fibres are then foamed using conventional rotor/stator systems (e.g. Hansa-Mixer, Mondo-Mix). The conventional foaming systems are therefore not suitable for foaming formulations containing fibres, since the fibres are deposited on and collect around the pins (fingers) of the rotors and around the pins (fingers) of the stators of the mixers, which then jam.
It is known from EP 1 289 638 B1 to disperse liquids with gases by deliberately causing cavitation.